1. Field of the Invention
The present invention relates to a photocopying apparatus, and particularly relates to such a copying apparatus provided with an automatic original conveyer for automatically conveying an original to a predetermined copying position.
2. Description of the Prior Art
When a large number of originals are to be photocopied at one time, a large burden is imposed on an operator. To make such copying work efficient, an automatic original conveyer has been used. An example of the automatic original conveyer will be described with reference to FIGS. 10-12.
In FIG. 10, an automatic original conveyer 10 includes an original tray 12 for receiving originals 11 therein, an original conveying portion 13 for conveying the original 11 fed from the original tray 12 to a copying position, an original conveying belt 16, and an original discharging roller 17.
The automatic original conveyer 10 is mounted on an upper glass surface 20. Scanning optical systems 21 and 22 for copying a picture carried by the original 11 are disposed under the glass surface 20. The scanning optical system 21 includes a lamp 21a and a mirror 21b so as to plane-scan the original 11 in the direction of an arrow 14 of FIG. 10 by the so-called "full-scanning". The scanning optical system 22, on the other hand, includes two mirrors 22a and 22b for scanning the original 11 in the direction of the arrow 14 by the so-called "half-scanning".
If copying is started with the originals 11 placed in the original tray 12 in the thus arranged automatic original conveyer 10, the first original 11 is fed between the glass surface 20 and the original conveying belt 16 through the original conveying portion 13. The original 11 is delayed at a gate 15 immediately before the original conveying belt 16. As soon as the gate 15 is opened, the original conveying belt 16 commences to be driven.
The original conveying belt 16 is driven for a predetermined period of time after the gate 15 has been opened so as to convey the original 11 to a copying position along the upper surface of the glass surface 20. Upon completion of conveyance of the original 11, the scanning optical systems 21 and 22 start scanning so as to perform copying. Upon completion of copying, the original conveying belt 16 conveys the original 11 to the discharging roller 17. Simultaneously with the discharge of the original 11, a succeeding original 11 is conveyed from the original tray 12 to the copying position.
By repetition of the operation as described above, the originals are successively conveyed one after another onto the glass surface 20 so that successive copying of those originals is carried out continuously and automatically.
The foregoing automatic original conveyer 10 can convey originals of various sizes.
In FIG. 11, an original 1a having a size, for example, of 11 .times.14 inches has been fed into a predetermined copying position on the glass surface 20. In this case, the original 1a is conveyed along a path shown by line 2 of FIG. 11, and stopped at the predetermined copying position so that the forward end of the original 1a comes on a fixed reference line 51.
Here, the scanning optical system (in this case, it is assumed that the same scanning optical system 21 for performing full-scanning in FIG. 10 is used) performs scanning along a lower surface of the glass surface 20 from the left end to the right end as shown by arrow 3 in FIG. 11, and returns to an original position after completion of scanning. For example, when copying of a plurality of sheets is required, each sheet is scanned in direction 3a and returned in direction 3b. This process is repeated several times as shown in FIG. 11.
If an 8.times.11 inch original 1b is copied by using the same automatic original conveyer 10 as shown in FIG. 12, the original 1b is conveyed onto the glass surface 20 as shown by arrow 2 in FIG. 12, and stopped at a predetermined copying position so that the forward end of the orginal 1b comes on the fixed reference line 51, in the same manner as the foregoing case of FIG. 11. The respective forward ends of the original 1a having a size of 11.times.14 inches in FIG. 11 and the original 1b having a size of 81/2.times.11 inches in FIG. 12 come on the same position.
In the case of FIG. 12, the scanning optical system performs scanning along the lower surface of the glass surface 20 from the left end to the right end of the original 1b as shown by an arrow 3' of FIG. 12 so as to repeat scanning in direction 3'a and returning in direction 3'b which are shorter than the scanning direction 3a and returning direction 3b in the case of FIG. 11.
In a copying apparatus in which copying of such a large number of originals is carried out at a high speed, a belt-like photosensitive element 25, for example, as shown in FIG. 13 is used. The belt-like photosensitive element 25 is guided by three guide rollers 26 so as to be always conveyed in the direction of an arrow 25a of FIG. 13. The effect is similar to an endless belt at a predetermined speed during a copying operation.
Generally, such a belt-like photosensitive element 25 has a seam portion 25b. Accordingly, an electrostatic latent image forming position is generally provided on the belt-like photosensitive element 25 so as to perform exposure while avoiding the seam port 25b because a normal electrostatic latent image cannot be formed on the seam portion 25b even if exposure is performed on the seam portion 25b. In this case, a reference mark 25c is put, for example, in the vicinity of the seam portion 25b of the photosensitive element 25, and exposure is started from a position shown by an arrow 19 at a predetermined timing after detection of the reference mark 25c by a reference mark sensor 25d.
Now, assume that the photosensitive element 25 is developed after separation at the seam portion 25b, for example, as shown in FIG. 14. Then, two electrostatic latent images 25e each forced upon exposure of an 11.times.14 inch original are obtained as illustrated in the drawing. Assuming that the photosensitive element 25 is being conveyed in the direction of an arrow 25a of FIG. 14, scanning 1 over the portion of the right electrostatic latent image 25e is carried out by the scanning optical system described above. The scanning optical system returns at the portion between the two electrostatic latent images 25e, while the next original is conveyed to the predetermined copying position in this period. Then, scanning 2 is carried out.
Here, when the cases of FIGS. 11 and 12 are compared with each other again, an original conveying distance of FIG. 11 is equal to that of FIG. 12 as shown by the arrows 2 although the size of the original 1a of FIG. 11 is different from that of the original 1b of FIG. 12. Therefore, in the automatic original conveyer, time taken for feeding an original from the original tray into the predetermined copying position in the case of FIG. 11 is equal to that in the case of FIG. 12.
In FIG. 14, in the period from termination of the scanning 1 to start of the scanning 2, the scanning optical system is returned, the original which has been copied is discharged, and the succeeding original is fed into the copying position.
Actually, scanning of the scanning optical system is started after the succeeding original has been fed into the predetermined copying position because return of the scanning optical system can be completed generally at a relatively high speed.
In so-called continuous copying in which one and the same original is repeatedly copied, on the other hand, the scanning optical system can perform scanning in a shorter period because a waiting period for conveying an original is omitted. If the length of the photosensitive element 25 and existence of the seam portion 25b are taken into consideration, however, the allotment of latent images is limited to that as shown in FIG. 14, with respect to an 11.times.14 inch original.
FIG. 15 shows an example of spaced electrostatic latent image forming positions with respect to an 81/2.times.11 inch original.
On the assumption that continuous copying is carried out at a short scanning distance as shown in FIG. 12, the spacing of FIG. 15 is selected so that the copying can be performed at the highest speed possible. In this case, electrostatic latent images 25e are formed at four positions corresponding to the respective scanning 1, scanning 2, scanning 3, and scanning 4, so that four copies should be obtained in every one complete revolution of the photosensitive element 25.
In the case where the original which has been copied is discharged after the scanning 1, and the succeeding original is conveyed, however, the time required for exchanging 1 as shown in FIG. 15 results in a loss of the timing for starting the scanning 2. Generally, since the scanning sequence of the scanning optical system is fixed at the point when the size of an original is detected, it is difficult to delay the starting timing of the scanning 2 to be synchronous with the timing of the exchanging 1. Consequently, after the exchanging 1, the starting of the scanning 3 is performed, by passing the sequence of scanning 2.
The scanning 4 should commence after the scanning 3 in the case of continuous copying. If the original exchanging 2 is performed here, however, the sequence of the scanning 4 is bypassed, and the start of scanning 1 is performed.
That is, although four copies can be obtained in every one complete revolution of the photosensitive element in continuous copying, only two copies can be obtained in every one complete revolution of the photosensitive element in the case where originals are exchanged every time copying is made.
This is because of the long time required for exchanging originals. As a result, there has been a disadvantage in that the productivity of the scanning optical system is reduced by half despite the difference between the time for returning of the scanning optical system and the time for exchanging originals being very small.
It is necessary to increase the conveying speed of the automatic original conveyer in order to solve the foregoing problem. However, an increase in conveying speed causes various problems which thereby increase the cost.
Further, although a change in the scanning sequence when originals are exchanged has been considered, no significant improvement is obtained because the copying operation is made more complicated. The change of the scanning sequence is therefore not practical.
It is therefore an object of the present invention to eliminate the foregoing disadvantages in the prior art.
It is another object of the present invention to provide a copying apparatus in which productivity can be improved without increasing the original conveying speed of an automatic original conveyer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the claims.